IMO, there may be a time when causing injury may prevent further injury to yourself or others. (I'm thinking about multiple armed attackers, for instance.) In this situation, breaking an attacker's arm with shihonage (for instance) would be preferable to a knife in your ribs, so the injury should be carried out as quickly and efficiently as possible in order to properly turn and defend against other attackers.

That being said, all possible care should be taken that an attacker isn't injured; deliberate injury should be committed only when there is no other choice.